EP1253117B1

EP1253117B1 - Glass substrate for photomasks and preparation method

Info

Publication number: EP1253117B1
Application number: EP02252784A
Authority: EP
Inventors: Masaki Shin-Etsu Chemical Co. Ltd. Takeuchi; Yukio Shin-Etsu Chemical Co. Ltd. Shibano
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 2001-04-20
Filing date: 2002-04-19
Publication date: 2011-01-12
Anticipated expiration: 2022-04-19
Also published as: JP2002318450A; US20020155361A1; DE60238885D1; US6869732B2; JP3627805B2; EP1253117A1

Description

This invention relates to glass substrates for use as photomasks in the most advanced application among semiconductor-related electronic materials, and a method for preparing the same.

BACKGROUND

Quality parameters of photomask-forming silica glass substrates include the size and density of defects, flatness, surface roughness, the opto-chemical stability of material, and the chemical stability of surface. More strict requirements are imposed on these parameters as the design rule becomes ultra-fine. In connection with the flatness of photomask-forming silica glass substrates, it is required to provide a glass substrate having not only a flatness in the acceptable range, but also a sufficient shape or topography to ensure that an exposure surface of a photomask prepared therefrom is flat during exposure. If the exposure surface is not flat during exposure, the exposure light suffers off-focusing on a silicon wafer to worsen the pattern uniformity, failing to form a precise micropattern.
The exposure surface of the photomask (surface subject to light exposure during an exposure step) is required to have a flatness of up to 2.2 nm per square centimeter of the exposure surface area, or up to 0.5 µm for the 6025 size (152 mm by 152 mm by 6.35 mm) photomask. Typically, the photomask is prepared by forming a light-shielding film on a silica glass substrate and patterning the film. On use, the photomask is most often set horizontally during the exposure step by holding the peripheral surface of the photomask by means of a suction grip or the like. Then, the shape of the photomask during the exposure step is governed by not only the flatness of the substrate, but also the resultant force of the film stress, gravity warp and holding force. These factors differ depending on the quality and thickness of the light-shielding film, the patterning mode and the exposure apparatus. The flatness of the exposure surface is largely affected by the flatness of the hold surface of the photomask substrate during the exposure step although this is not always the case. It would be desirable that the portion of the silica glass substrate corresponding to the hold surface of the photomask have a higher degree of flatness. Specifically, the portion of the silica glass substrate corresponding to the hold surface of the photomask should preferably have a flatness of up to 1.3 nm/cm². Such local leveling is, in fact, impossible with current leveling methods based on the traditional polishing technology.
Takeuchi et al. "Properties of Our Developing Next Generation Photomask Substrate" (Proc. SPIE - Int. Soc. Opt. Eng (USA), 1999 (3748), 41-52) describes the development of a method of producing synthetic silica glass photomask substrates with a smooth surface (no defects larger than 0.3 µm) and high global flatness (0.5 µm).
Page 682 of the Research Disclosure Journal (No. 340, 1 August 1992), "Phase Shift Mask Fabrication Using Reactive Ion Etching of Quartz Substrates" describes reactive ion etching to dry etch quartz structures to a mean phase depth of interest.
US 5,225,035 describes a method of etching a phase-shift photomask such that the light transmittance is identical in all transparent regions of the mask, whether etched or unetched.
If the entire surface of a substrate is made flat, its portion corresponding to the hold surface is also flat. In the existing precision polishing methods such as rotary double-sided lapping on a batchwise basis and rotary single-side lapping on a single-wafer basis, soft abrasive cloths are used in the polishing of substrates so as to incur no or few defects. It is, in fact, impossible to level out nonuniformities over the entire substrate surface including its periphery.
When the factors associated with film stress and gravity warp must be taken into account, a technique capable of establishing a more complex arbitrary shape is needed, and the traditional precision polishing technology comes to the limit of its abilities.
An object of the invention is to provide a glass substrate for a photomask, which is configured such that an exposure surface of the photomask or the hold surface of the substrate becomes flat in an exposure step; and a method for preparing the same.
The invention is predicated on the following discovery. When a glass substrate having a patterned light-shielding film on a surface thereof is used as a photomask, the photomask is held in an exposure apparatus and exposed to light. The photomask includes a surface which is subject to exposure (exposure surface, hereinafter) and should be flat during the exposure step; or a portion of the glass substrate corresponding to the held portion of the photomask in the exposure apparatus should be flat. By computing a difference between the shape of the glass substrate having a patterned light-shielding film wherein the exposure surface is flat during the exposure step or the portion of the glass substrate corresponding to the held portion of the photomask in the exposure apparatus is flat and the shape of a starting glass substrate, and carrying out local plasma etching on the surface of the starting glass substrate in accordance with the computed difference, there is obtained a photomask-forming glass substrate having such a shape or topography that the exposure surface during the exposure step may have a flatness of 0.04 nm to 2.2 nm per cm² of the exposure surface area, or the portion of the glass substrate corresponding to the held portion of the photomask in the exposure apparatus (sometimes referred to as hold surface) may have a flatness of 0.04 nm to 4.5 nm per cm² of the hold surface area.
In one aspect the invention provides a photomask comprising a glass substrate and a patterned light-shielding film, the light-shielding film being on a film-receiving surface of the glass substrate to form an exposure surface of the photomask, and the photomask being for mounting in exposure apparatus in which the exposure surface is exposed to light to pattern a silicon wafer, characterised in that: before having the light-shielding film thereon and before mounting in the exposure apparatus, the film receiving surface of the glass substrate has a predetermined contoured target shape, shaped by plasma etching selected so that deformation of the glass substrate caused by the presence of the light-shielding film and mounting in the exposure apparatus gives the exposure surface a flatness in use of 0.04 nm to 2.2 nm per square centimetre.
In a further aspect, the invention provides a glass substrate for a photomask which has a light-shielding film on a film-receiving surface of the glass substrate to form an exposure surface of the photomask, and which is for mounting in exposure apparatus in which the exposure surface is exposed to light to pattern a silicon wafer, wherein the glass substrate includes a mounting region corresponding to a held region of the photomask receivable by the exposure apparatus, characterised in that:

the mounting region is shaped by plasma etching to have a surface flatness of from 0.04 nm to 4.5 nm per square centimetre.

The invention also provides a method of preparing a glass substrate for a photomask, the photomask comprising the glass substrate and a light-shielding film, the light-shielding film being on a film-receiving surface of the glass substrate to form an exposure surface of the photomask, and the photomask being for mounting in exposure apparatus in which the exposure surface is exposed to light to pattern a silicon wafer, the method including:

determining a difference between a predetermined target contour and an initial surface contour of the glass substrate; and
in dependence on the determined difference, carrying out region-selective plasma etching of the glass substrate to approximate the initial surface contour to the predetermined target contour in order that the photomask is flat in use.

BRIEF DESCRIPTION OF THE DRAWING

The only figure, FIG. 1 is a contour line diagram showing the ideal shape of a glass substrate in Example 1.

FURTHER EXPLANATIONS; OPTIONS AND PREFERENCES

The invention employs a plasma etching technique as the means for producing glass substrates for high-flatness photomasks. According to the invention, a difference between the shape of the final glass substrate wherein the exposure surface is flat during the exposure step and the shape of a starting glass substrate is computed. Local plasma etching is then carried out on the surface of the starting glass substrate in accordance with the computed difference, that is, at surface spots or regions to be removed. Namely, plasma etching treatment is carried out while locally changing the amount of etching so as to increase or decrease the etching amount in accordance with the shape of a portion of a starting glass substrate as compared with the ideal shape of a corresponding portion of the final glass substrate wherein the exposure surface is flat during the exposure step. As for a portion of the photomask which is held in the exposure apparatus, a difference between the shape of the final glass substrate wherein a portion of the glass substrate corresponding to the held portion is flat and the shape of a starting glass substrate is computed, and local plasma etching is carried out in accordance with the difference.
As used herein, the phrase that the exposure surface or the hold surface is flat means that the surface is flat in the state where a patterned light-shielding film is present on a glass substrate. A photomask is generally prepared by forming a light-shielding film on a photomask-forming glass substrate, applying a resist film thereon, irradiating light to selected regions of the resist film, developing the resist film for patterning, etching the light-shielding film in accordance with the resist film pattern, and finally removing the resist film, leaving the patterned light-shielding film on the glass substrate. The photomask is loaded and held in an exposure apparatus where light exposure is carried out through the photomask for transferring its pattern to a wafer. When it is described that the exposure surface is flat during the exposure step or the portion of the photomask which is held in the exposure apparatus is flat, it refers to a flatness in the state where the patterned light-shielding film is present on the glass substrate at the end of manufacture of the photomask in the above-described manner. Then the ideal shape means that the exposure surface or the hold surface is completely flat in said state.
When plasma etching is carried out in accordance with the computed shape difference as described above, a plasma generating housing is positioned above a starting glass substrate at a surface spot to be removed, and an etchant gas is flowed through the housing. Neutral radical species generated in the plasma isotropically attack the glass substrate surface at that spot whereby the spot is etched away. In areas of the glass substrate surface outside the plasma generating housing, no plasma is generated and no etching takes place even though some etchant gas strikes against the surface. When the plasma generating housing is moved above the starting glass substrate, the rate of movement of the housing is controlled in accordance with the necessary amounts of removal from the starting glass substrate surface so that a glass substrate having the desired shape is eventually obtainable.
It is necessary that the surface shape or topography of the starting glass substrate be previously determined. Surface shape measurement may be done by any methods. It is desired that such measurement be of high precision, and thus, an optical interference method is a typical measurement. In accordance with the surface shape of the starting glass substrate, the rate of movement of the plasma generating housing is computed. Then the rate of movement is controlled to be slow in an area requiring a large amount of removal, so as to provide a more etching amount.
The photomask is prepared by first forming a light-shielding film of chromium or the like on a glass substrate, applying a resist film thereon, and writing a desired pattern on the resist film using an electron beam or the like. The resist film is then developed and the light-shielding film is etched therethrough. In this way, the photomask including light-transmitting and shielding areas is obtained. The photomask is set in an exposure apparatus where light exposure is carried out through the photomask to a silicon wafer having a resist film coated thereon, followed by a conventional device fabrication process.
In order that plasma etching be used to produce a photomask-forming glass substrate which is configured such that the exposure surface is flat during the exposure step, the ideal shape is computed from the conditions under which the photomask is set during the exposure step, a difference between the ideal shape and the shape of a starting substrate being a necessary amount of material to be removed by plasma etching. Also, the flatness of the portion of the glass substrate corresponding to the held portion of the photomask during the exposure step is important as previously described, and in this case, a difference between the shape of a portion of the starting substrate surface corresponding to the held portion and the ideal flat shape of the same portion gives a necessary amount of material to be removed.
The ideal shape can be determined from the type of exposure apparatus, the quality and thickness of the light-shielding film and the patterning mode, by stress computing simulation. Specifically, the surface area and flatness of a portion of a glass substrate corresponding to the held portion are determined from the shape of a photomask holder in the exposure apparatus. The light-shielding film is formed, for example, of chromium, chromium oxide or molybdenum silicide, and the film stress to the glass substrate can be computed from the material, structure and thickness of the light-shielding film. The light-shielding film may be a single layer or a plurality of layers. From the position, shape and area of a pattern on the film surface, the stress distribution of the patterned film that deforms the entire glass substrate can be computed. Finally, the ideal shape of the glass substrate which is configured such that the exposure surface is flat during the exposure step is determined by a counting-back procedure with any warp by its own weight of the photomask set in the exposure apparatus being additionally taken into account.
On the other hand, the shape or flatness of the starting glass substrate is measured by means of a flatness meter of the optical interference system as mentioned above. An amount of material corresponding to its difference from the ideal shape is removed by plasma etching.
The plasma generating housing may be of any structure. In one exemplary system, the glass substrate is sandwiched between a pair of electrodes, a plasma is created between the substrate and the electrode by application of a radio frequency power, and an etchant gas is flowed therethrough to generate radical species. In another system, an etchant gas is passed through a waveguide where a plasma is created by microwave oscillation, and the flow of radical species thus generated is impinged against the substrate surface.
The etchant gas is selected depending on the identity of glass substrate. For silica glass substrates for photomasks, a halide gas or a gas mixture containing a halide gas is preferred. The halide gas is exemplified by methane tetrafluoride, methane trifluoride, ethane hexafluoride, propane octafluoride, butane decafluoride, hydrogen fluoride, sulfur hexafluoride, nitrogen trifluoride, carbon tetrachloride, silicon tetrafluoride, methane trifluoride chloride and boron trichloride.
To control the rate of movement of the plasma generating housing in accordance with the amount of material to be removed from the surface of the starting glass substrate as mentioned above, a computer may be used. Since the movement of the plasma generating housing is relative to the substrate, the substrate itself may be moved instead.
The glass substrate is thus processed such that the exposure surface or the hold surface of the resultant photomask has a very high degree of flatness during the exposure step. The desired flatness of the exposure surface is 0.04 nm to 2.2 nm, especially 0.04 nm to 0.86 nm per square centimeter of the exposure surface. In the case of 6025 substrates (152 mm by 152 mm by 6.35 mm) which are most often used as the photomask-forming glass substrate, the exposure surface preferably has a flatness of 0.01 µm to 0.5 µm, especially 0.01 µm to 0.2 µm during the exposure step. The portion of the glass substrate corresponding to the portion of the photomask which is held in the exposure apparatus during the exposure step preferably has a flatness of 0.04 nm to 4.5 nm, especially 0.04 nm to 2.2 nm per square centimeter of the hold surface. The glass substrate is preferably dimensioned 152 ± 0.2 mm by 152 ± 0.2 mm by 6.35 ± 0.1 mm.
Under certain plasma etching conditions, the surface of the glass substrate thus obtained can have surface roughness or a work-degraded layer. In such a case, if necessary, the plasma etching may be followed by a very short time of polishing which does bring substantially little change of flatness.
Preferred for the measurement of flatness/unevenness considering measurement precision is an optical interference method in which coherent light (matched phase relationship) as typified by laser light is directed to the substrate surface and reflected thereby, and a difference in height of substrate surface points is observed as a phase shift of reflected light.
The invention provides a glass substrate which is configured such that the exposure surface or the hold surface of the resulting photomask is fully flat during the exposure step. It is suited for use as silica glass substrates for photomasks used in the photolithography of great interest in the fabrication of ICs. The invention thus contributes to the achievement of finer patterns in the semiconductor field.

EXAMPLE

Examples of the invention are given below by way of illustration and not by way of limitation.

Example 1

The starting substrate used was a quartz substrate having a pair of square surfaces of 152 mm by 152 mm and a thickness of 6.4 mm. Plasma etching was carried out thereon. A photomask was obtained from the processed substrate. A suction grip was used to hold the photomask in an exposure apparatus, with the held portion having an area of 152 mm × 3.5 mm (rectangular shape). The light-shielding film had a three-layer structure of chromium with a thickness of 400 nm and was patterned in a 1:1 line-and-space pattern with a line width of 0.80 µm. By stress computing simulation, the ideal shape of a photomask-forming quartz substrate was determined as shown in FIG. 1. The ideal flatness was 0.43 nm/cm². The surface shape of the starting substrate was measured by an optical interference flatness meter, finding a flatness of 3.4 nm/cm². A difference of this shape from the ideal shape was computed, a necessary amount of material to be removed was determined therefrom, and plasma etching was accordingly carried out. The plasma generating housing of radio frequency type (150 W) included cylindrical electrodes of 75 mm diameter. The etchant gas used was methane tetrafluoride. After plasma etching was carried out on the substrate, the substrate surface was measured again by the optical interference flatness meter, finding a shift of only 2% from the ideal shape. The substrate was then lightly polished by a single-side lapping machine and cleaned, whereupon the light-shielding film described above was coated and patterned. The light-shielding film-bearing substrate was customarily set in the same state as the photomask was held in the exposure apparatus. In this state, measurement by the optical interference flatness meter revealed that the exposure surface had a flatness of 0.1 nm/cm².

Example 2

The starting substrate used was a quartz substrate having a pair of square surfaces of 152 mm by 152 mm and a thickness of 6.4 mm. The shape of the quartz substrate was measured by an optical interference flatness meter, finding a flatness of 3.9 µm. Based on the irregularity data of regions of this substrate surface corresponding to the portions of the photomask to be held during the exposure step (two rectangular regions of 152 mm × 3.5 mm at the substrate periphery sides), a necessary amount of material to be removed from these regions was determined. Plasma etching was carried out on the substrate surface in accordance with that amount.
The plasma generating housing of radio frequency type (150 W) included cylindrical electrodes of 75 mm diameter. The etchant gas used was methane tetrafluoride. After plasma etching was carried out on the substrate regions corresponding to the (photomask) held portions, the substrate surface was measured again by the optical interference flatness meter, finding that the rectangular regions of 152 mm × 3.5 mm corresponding to the (photomask) held portions had a flatness of 2.1 nm/cm².
Reasonable modifications and variations are possible from the above-described embodiments in the light of the general teachings herein.

Claims

A photomask comprising a glass substrate and a patterned light-shielding film, the light-shielding film being on a film-receiving surface of the glass substrate to form an exposure surface of the photomask, and the photomask being for mounting in exposure apparatus in which the exposure surface is exposed to light to pattern a silicon wafer,
characterised in that;
before having the light-shielding film thereon and before mounting in the exposure apparatus, the film receiving surface of the glass substrate has a predetermined contoured shape, shaped by plasma etching selected so that deformation of the glass substrate caused by the presence of the light-shielding film and mounting in the exposure apparatus gives the exposure surface a flatness in use of 0.04 nm to 2.2 nm per square centimetre.
A glass substrate for a photomask which has a light-shielding film on a film-receiving surface of the glass substrate to form an exposure surface of the photomask, and which is for mounting in exposure apparatus in which the exposure surface is exposed to light to pattern a silicon wafer,
wherein the glass substrate includes a mounting region corresponding to a held region of the photomask receivable by the exposure apparatus,
characterised in that:
the mounting region is shaped by plasma etching to have a surface flatness of from 0.04 nm to 4.5 nm per square centimetre.
As photomask or glass substrate according to claim 1 or 2, wherein the glass substrate is silica glass.
Use of a glass substrate having said predetermined contoured target shape as specified in claim 1 or mounting region as specified in claim 2 in the manufacture of a photomask according to claim 1 or 3.
A method of preparing a glass substrate for a photomask, the photomask comprising the glass substrate and a light-shielding film, the light-shielding film being on a film-receivihg surface of the glass substrate to form an exposure surface of the photomask, and the photomask being for mounting in exposure apparatus in which the exposure surface is exposed to light to pattern a silicon wafer, the method including:
determining a difference between a predetermined target contour and an initial surface contour of the glass substrate; and

in dependence on the determined difference, carrying out region-selective plasma etching of the glass substrate to approximate the initial surface contour to the predetermined target contour in order that the photomask is flat in use.
A method of preparing a glass substrate according to claim 5, wherein the region-selective plasma etching shapes the film-receiving surface of the glass substrate to give a flat exposure surface in use.
A method of preparing a glass substrate according to claim 5 or 6, wherein the region-selective plasma etching shapes a mounting region surface of the glass substrate which corresponds to a held region of the photomask receivable by the exposure apparatus to give a flat held region of the photomask before mounting in the exposure apparatus.
A method of preparing a glass substrate according to any of claims 5 to 7, wherein the target contour for the glass substrate is determined taking into account any one or more of:
(a) stress in the light-shielding film tending to deform the substrate;

(b) variation in (a) associated with a patterning or intended patterning of the light-shielding film;

(c) deformation caused by the mounting of the substrate in intended exposure apparatus;

(d) gravity warp when mounted in intended exposure apparatus.
A method of preparing a glass substrate according to any of claims 5 to 8, wherein the initial surface contour is determined using optical interference scanning.
A method of preparing a glass substrate according to any of claims 5 to 9, using computer control of the region-selective plasma etching process in dependence on the determined difference between the initial surface contour and stored data defining the predetermined target contour.